The typical electrical power supply system for automotive vehicles is a so-called constant voltage system having a nominal rated voltage of 12 volts. It comprises, in general, a storage battery, a DC generator and a regulator to maintain the generator output voltage and current within certain limits. DC generator, which typically takes the form of an alternator with a rectifier, is driven at variable speed by the vehicle engine and supplies the DC power to charge the battery. The regulator is typically a switching device for alternately connecting and disconnecting the field coil of the alternator across the battery when the output voltage of the generator is respectively below and above a desired value and for disconnecting the field coil from the battery when the generator current exceeds a predetermined value.
The typical automotive electrical system has become somewhat complex with more and a wider variety of load devices. The conventional 12 volt system has certain disadvantages in supplying the various load devices. One disadvantage is that certain devices such as air blowers and window heaters which require relatively large amounts of power must be connected through large wires because of high current requirements. Another disadvantage is that certain load devices otherwise suited to automotive applications, such as metal-film type window heaters, require higher voltage than the conventional 12 volt supply. Further, power semi-conductor devices are inherently much less efficient and/or more expensive for controlling power at 12 volts than for controlling the same power at higher voltage such as 24 or 40 volts.
The prior art relating to vehicular electrical systems is replete with dual voltage power supplies of various forms.
The electrical systems of certain large trucks utilize a 24 volt cranking motor while the other accessory devices are rated at 12 volts. In such systems, two 12 volt batteries are provided which are connected in series for supplying the cranking motor and are connected in parallel for charging to 12 volts by the generator and for supplying the 12 volt load devices. Various arrangements have been proposed to eliminate the need for the series-parallel switching arrangement in such dual voltage systems. In the Raver U.S. Pat. No. 3,962,621, a dual battery charging generator system is described for 12 and 24 volt batteries. In this system, the alternator output is rectified and used to charge the 12 volt battery whereas the alternator output is transformed to a higher voltage and rectified to charge the 24 volt battery. In the Baumgartner et al U.S. Pat. No. 3,793,544, a dual voltage system is described in which an alternator is provided with a pair of stator windings and respective rectifiers to provide two different output voltages. In the Taylor U.S. Pat. No. 4,210,856, a dual voltage system is provided with two different batteries. One battery is charged from an alternator-rectifier and the two batteries in series are charged from a DC-to-DC converter energized from the first battery. Another dual voltage, dual battery system is disclosed in the Himmler U.S. Pat. No. 4,047,088. In this system, the output of the three-phase alternator-rectifier is used to charge two 12 volt batteries which are connected in series across the 24 volt load circuit. One of the batteries, which is connected with the 12 volt circuit, is charged by an auxiliary charging circuit which is connected across one of the phase windings of the alternator. In the Seike U.S. Pat. No. 3,710,226, a dual voltage system with two batteries is disclosed in which the two batteries are connected in series. One of the batteries is charged by two cooperating phases of the alternator, the output of which is controlled by a field regulator for the alternator. The other battery is charged by the third phase of the alternator, the output of which is regulated by a series regulator. In the Follmer U.S. Pat. No. 4,044,293, a dual voltage system with two batteries is described in which the alternator charges the series combination of the batteries. A separate regulator is provided for one of the batteries and when it is fully charged, the regulator is effective to bypass that battery out of the circuit so that the alternator is connected only with the first battery.
A general object of this invention is to provided an improved dual voltage power supply system for automotive vehicles which overcomes certain disadvantages of the prior art.